COBE

COBE Slide Set

High-resolution digital images are available at slide set jepg files if you need a high-resolution digital image for publication.

Slide Captions

Slide
1: Early development of the Universe.
The Cosmic Background Explorer (COBE)
satellite was designed to measure the
diffuse infrared and microwave radiation
from the early Universe, to the limits
set by our astrophysical environment.
The COBE was developed by NASA's
Goddard Space Flight Center with
scientific guidance from the
COBE Science Working Group.

Slide
2: Artist's conception of the COBE
satellite in orbit, annotated with locations
of scientific instruments, dewar, etc.
The instruments are the Far Infrared
Absolute Spectrophotometer (FIRAS),
which made a precise measurement of
the spectrum of the cosmic microwave
background radiation; the Differential
Microwave Radiometers (DMR), which
detected for the first time and was
used to characterize faint fluctuations
in the cosmic microwave background corresponding
to density structure in the early Universe;
and the Diffuse Infrared Background
Experiment (DIRBE), which obtained
data that can be used to seek the cosmic
infrared background and study the structure
of the Milky Way Galaxy and the interstellar
and interplanetary dust. The COBE was
launched on November 18, 1989. All three
instruments performed well while the
helium cryogen supply lasted, until
September 21, 1990. Thereafter, the
FIRAS ceased operating, as did the DIRBE
at wavelengths longer than 4.9 µm
(micrometers, or "microns").
However, the DMR continued to operate
normally, and the DIRBE continued to
collect near-infrared data with diminished
sensitivity until these instruments
were finally turned off on December
23, 1993.

Slide
3: The COBE orbit and spin axis
orientation. The orbit nearly passes
over the Earth's poles at an altitude
of 900 km (559 miles). The orbital plane
is inclined by 99 degrees to the Equator,
causing the orbit to precess (turn)
to follow the apparent motion of the
Sun relative to the Earth. (The precession
is caused by the Earth's equatorial
bulge, which in turn results from the
Earth's daily rotation about its axis.)
Thus, the spin axis stays pointed almost
perpendicular to the direction of the
Sun and in a generally outward direction
from the Earth. As the COBE orbits the
Earth once every 103 minutes, it views
a circle on the sky 94 degrees away
from the Sun, and as the Earth moves
around the Sun over the course of a
year the COBE gradually scans the entire
sky. The spacecraft rotates at 0.8 rpm.
The FIRAS instrument is aligned with
the spin axis. The DIRBE and DMR instruments
point "off axis" and observe
half the sky every orbit.

Slide
4: DIRBE optical concept showing
mirrors, filters, detectors, and beam
interrupter. The DIRBE uses an unobscured
off-axis Gregorian telescope to collect
light and bring it to a focus on 16
infrared detectors. The instrument was
designed to minimize response to objects
outside the desired 0.7 degree square
field of view. The vibrating beam interrupter
allows continuous comparison of the
sky brightness with the cold (2 degree
Kelvin) interior of the instrument.
The responsivity of each detector is
monitored on a regular basis using the
on-board internal reference sources.
The same square field of view is observed
simultaneously at each of ten wavelengths
(1.25, 2.2, 3.5, 4.9, 12, 25, 60, 100,
140, and 240 µm), and polarization
is measured at the three shortest wavelengths.

Slide
6: DIRBE scan track superposed on
100 µm Annual Average Map and
100 µm intensity from the corresponding
segment of time-ordered data. DIRBE
scanned the sky in a helical pattern
that resulted from the spin and orbital
motion of the COBE satellite and the
"look direction" of the telescope,
which was 30 degrees from the spin axis.
The scan segment depicted covers two
COBE spin cycles, or about 150 seconds,
during a time when the DIRBE field of
view swept through the Sco-Oph region
(bright area above the Galactic center
in the figure) and passed near the North
Galactic Pole, where the emission is
faint. The brightness of the sky at
100 µm measured during this interval
is shown as a graph of intensity vs.
time, as given in the
DIRBE Time-ordered Data product.
On the abscissa, the unit of time is
1/8th of a second. The bumps at about
470 and 1040 time units correspond to
ecliptic plane crossings; zodiacal emission
is at a maximum in the ecliptic plane.

Slide 7: 100 µm
Weekly Sky Maps for mission weeks
4 to 44, and the 100 µm
Annual Average Map. Shows sky coverage
each week of the DIRBE mission over
the period during which the COBE cryogen
supply lasted. As the Earth, with COBE
in orbit, revolved around the Sun, DIRBE
viewed the sky from an ever-changing
vantage point in the solar system, enabling
light reflected and emitted by the interplanetary
dust cloud to be modeled.

Slide
8:
Annual Average Maps at 3.5, 25,
100, and 240 µm. Galactic coordinate
Mollweide projection maps of the entire
sky at four wavelengths showing emission
from stars and dust in the Galactic
plane (horizontal feature) and light
scattered and emitted by dust in the
solar system (S-shape).

Slide
9: 1.25, 2.2, and 3.5 µm
Solar elongation angle = 90 degree Maps.
Galactic coordinate Mollweide projection
maps of the entire sky as seen by the
DIRBE at a fixed angle relative to the
Sun. Stars concentrated in the Galactic
plane (horizontal feature) dominate
the images at these wavelengths. Dust
in the Milky Way absorbs and scatters
starlight, producing the dark band that
runs through the Galactic center in
the 1.25 µm image; this "extinction"
effect diminishes with increasing wavelength.

Slide
10: False-color image of the near-infrared
sky as seen by the DIRBE. Data at 1.25,
2.2, and 3.5 µm wavelengths are
represented respectively as blue, green
and red colors. The image is presented
in Galactic coordinates, with the plane
of the Milky Way Galaxy horizontal across
the middle and the Galactic center at
the center. The dominant sources of
light at these wavelengths are stars
within our Galaxy. The image shows both
the thin disk and central bulge populations
of stars in our spiral galaxy. Our Sun,
much closer to us than any other star,
lies in the disk (which is why the disk
appears edge-on to us) at a distance
of about 28,000 light years from the
center. The image is redder in directions
where there is more dust between the
stars absorbing starlight from distant
stars. This absorption is so strong
at visible wavelengths that the central
part of the Milky Way cannot be seen.
DIRBE data will facilitate studies of
the content, energetics and large scale
structure of the Galaxy, as well as
the nature and distribution of dust
within the Solar System. The data also
will be studied for evidence of a faint,
uniform infrared background, the residual
radiation from the first stars and galaxies
formed following the Big Bang.

Slide
11: 1.25, 2.2, 3.5 µm composite
image of Galactic center region. Shows
asymmetric shape of the bulge at the
center of the Milky Way. The image is
a Mollweide projection covering 60 degrees
in Galactic longitude by 20 degrees
in Galactic latitude and centered on
the Galactic center.

Slide
12: 4.9, 12, 25, and 60 µm
Solar elongation angle = 90 degree Maps.
Thermal emission from star-heated dust
in the Milky Way and interplanetary
dust heated by the Sun dominates the
images at these wavelengths. The S-shaped
feature is the ecliptic plane, in which,
like the planets, the interplanetary
dust is concentrated. The oval-shaped
brightness discontinuity is an artefact
of the way the maps were prepared, not
a feature in the infrared sky. The discontinuity
corresponds to a path difference through
the interplanetary dust cloud as adjacent
positions in the sky were observed from
DIRBE's vantage point in Earth orbit
with the Earth on opposite sides of
the Sun.

Slide
13: This image combines data from
the DIRBE obtained at infrared wavelengths
of 4.9, 12 and 25 µm. The sky
brightness at these wavelengths is represented
respectively by blue, green, and red
colors in the image. The plane of the
Milky Way Galaxy lies horizontally across
the middle of the image with the Galactic
center at the center. Emission from
interplanetary dust in our solar system
is very prominent, as shown by the orange
"S-shaped" curve which follows
the ecliptic plane. The thin lines forming
bands within the curve show the structure
of streaming dust grains - the result
of colliding asteroids. At these wavelengths,
the broad plane of the Milky Way, which
appears as blue and pink, reveals stars
and interstellar cool dust in the disk
of the Milky Way. To make the contributions
from the Solar System as uniform as
possible, the images are made from observations
when the Sun angle (solar elongation)
was 90 degrees from the viewing direction.
DIRBE is the first space instrument
designed to make a comprehensive sky
survey in the search for an ancient
fossil known as the cosmic infrared
background (CIB) radiation - the remnant
from the formation of the earliest objects
in the Universe created 5 to 12 billion
years ago. Extensive modeling is required
to isolate the CIB from the infrared
foregrounds from the Solar System and
Galaxy.

Slide
14: This image combines data from
the DIRBE obtained at infrared wavelengths
of 25, 60 and 100 µm. The sky
brightness at these wavelengths is represented
respectively by blue, green, and red
colors in the image. The plane of the
Milky Way Galaxy lies horizontally across
the middle of the image with the Galactic
center at the center. The image is dominated
by the thermal emission from interstellar
dust in the Milky Way. The wispy-looking
dust features are called "infrared
cirrus." The structured, warmer
emission from interplanetary dust, shown
in blue, is also prominent. The image
shows a number of well-known dusty regions
in the Milky Way, such as the Orion
molecular clouds (below the plane, far
right) which are active "stellar
nurseries" in our Galaxy. Two neighboring
galaxies, the Large and Small Magellanic
Clouds also can be distinguished (below
the plane, approximately halfway between
the center and right edge of the image).

Slide
15: 100, 140, and 240 µm
Solar elongation angle = 90 degree Maps.
Thermal emission from relatively cool
interstellar dust warmed by stars in
the Milky Way dominates at these wavelengths.
At high Galactic latitudes, interstellar
"cirrus" clouds are apparent.
Emission from the solar system dust
("zodiacal emission") is strongest
at 25 µm but remains in evidence
in the 100 µm image, and to a
lesser degree at the longer wavelengths.

Slide
16: DIRBE 1.25 and 2.2 µm
maps of the sky as observed (top) and
following subtraction of a detailed
model of the zodiacal light (middle
and bottom), which at these wavelengths
is Sunlight scattered by interplanetary
dust grains. The maps are Mollweide
projections in geocentric ecliptic coordinates.
In this projection, the Galactic plane
is seen as a bright arc across the sky,
and the zodiacal light is concentrated
in a horizontal band which runs through
the middle of the map. At each wavelength,
the top and middle maps are shown on
the same brightness scale; the maps
on the bottom are shown at a narrower
brightness range in order to accentuate
imperfections in the zodiacal light
model. These imperfections are most
evident at mid-infrared wavelengths
where the interplanetary dust signal
is strongest (12, 25, 60 µm).
Such defects notwithstanding, all of
the emission seen in the middle and
bottom maps is either Galactic (stars
in the Milky Way) or extragalactic in
origin. The main scientific goal of
the DIRBE is to measure the brightness
of the extragalactic component of this
residual emission, which is presumed
to be isotropic (i.e., uniform
over the sky). The DIRBE zodiacal
light model is described by Kelsall
et al. 1998, ApJ, in press.

Slide
17: DIRBE 3.5 and 4.9 µm maps
of the sky as observed (top) and following
subtraction of a detailed model of the
zodiacal light (middle and bottom).
At these wavelengths, "zodiacal
light" includes both thermal emission
from and Sunlight scattered by interplanetary
dust grains (see Kelsall et al.
1998, ApJ, in press). The Slide 16 caption
above contains information about the
map projection and brightness scales.
The residual emission at 3.5 and 4.9
µm (middle and bottom maps) comes
predominantly from stars in the Milky
Way. Imperfections in the zodiacal light
model are evident in the 4.9 µm
map on the bottom, where the brightness
scale was compressed in order to emphasize
low-brightness features.

Slide
18: DIRBE 12 and 25 µm maps
of the sky as observed (top) and following
subtraction of a detailed model of the
zodiacal light (middle and bottom),
which at these wavelengths is thermal
emission from interplanetary dust grains
heated by absorbed Sunlight (see Kelsall
et al. 1998, ApJ, in press).
The Slide 16 caption above contains
information about the map projection
and brightness scales. The residual
emission at 12 and 25 µm (middle
and bottom maps) comes predominantly
from interstellar dust in the Milky
Way which, like the interplanetary dust,
absorbs starlight and gives off thermal
emission. The emission from interstellar
dust is strongest at longer infrared
wavelengths (> 60 µm) than
the interplanetary dust emission
(peak emission at about 25 µm)
because typical interplanetary grains
are warmer than their interstellar counterparts.
This is because interplanetary grains
are relatively close to the Sun compared
to the distance between the average
interstellar grains and the nearest
stars. Imperfections in the zodiacal
light model are evident in both of the
bottom maps, where the brightness scale
was compressed in order to emphasize
low-brightness features.

Slide
19: DIRBE 60 and 100 µm maps
of the sky as observed (top) and following
subtraction of a detailed model of the
zodiacal light (middle and bottom),
which at these wavelengths is thermal
emission from interplanetary dust grains
heated by absorbed Sunlight (see Kelsall
et al. 1998, ApJ, in press).
The Slide 16 caption above contains
information about the map projection
and brightness scales, and the Slide
18 caption describes some of the relevant
astrophysics. Galactic interstellar
dust emission is quite strong at these
wavelengths, while zodiacal emission
is relatively weak, as can be seen in
the maps on the top. Imperfections in
the zodiacal light model are evident
at 60 µm but markedly less apparent
at 100 µm, as can be seen in the
maps on the bottom.

Slide
20: DIRBE 140 and 240 µm maps
of the sky as observed (top) and following
subtraction of a detailed model of the
zodiacal light (middle and bottom; see
Kelsall et al. 1998, ApJ, in
press). The Slide 16 caption above contains
information about the map projection
and brightness scales, and the Slide
18 caption describes some of the relevant
astrophysics. Galactic interstellar
dust emission is much stronger at these
wavelengths than zodiacal emission,
as can be seen in the maps on the top.
Even on a stretched brightness scale
(bottom maps), imperfections in the
zodiacal light model are not apparent.
Detector noise from the DIRBE instrument
is significant at these wavelengths
and gives rise to the speckled appearance
of the maps.

Slide
21: At near-infrared wavelengths,
following the subtraction of zodiacal
light (see Slide 16), map pixels containing
discrete bright sources are masked and
the DIRBE Faint Source Model is
used to subtract residual Galactic starlight
in order to detect or place an upper
limit on the brightness of the cosmic
infrared (extragalactic) background
emission (Arendt et al. 1998,
ApJ, in press). Here the upper map shows
the residual sky brightness at 2.2 µm
after zodiacal light subtraction and
bright source masking (dark spots in
maps). In this projection, the Galactic
plane runs horizontally through the
map. Ideally, if the zodiacal model
were perfect, only the collective emissions
of (faint) stars in the Milky Way and
the sought-after extragalactic light
(cosmic infrared background) would remain
in this map. The lower map shows the
DIRBE Faint Source Model. To facilitate
comparison, both maps are shown on the
same brightness scale and with the same
pixels masked. Clearly, most of the
residual 2.2 µm emission in the
upper map is attributable to stars in
the Milky Way.

Slide
22: Results of the DIRBE search
for the Cosmic Infrared Background (CIB)
after removal of foreground emissions
from the solar system and the Milky
Way (see Hauser et al. 1998,
ApJ, in press). The two black circles
with error bars represent DIRBE detections
of the CIB at 140 and 240 µm.
(These detections were the subject of
a press
release.) Circles with downward-pointing
arrows represent DIRBE 2 sigma upper
limits at 1.25 - 100 µm; the tips
of the arrows indicate the measured
residual values after foreground subtraction.
Prior to subtracting the foreground
emissions, DIRBE observations of the
darkest regions in the sky in each wavelength
band were interpreted as conservative
upper limits on the CIB brightness (hatched
horizontal lines). The other symbols
represent limits or tentative detections
of the CIB from non-DIRBE data
sources.

Slide
23: An illustration of the foreground
emission subtraction process resulting
in the DIRBE detection of the Cosmic
Infrared Background at 240 µm.
The map at the top is a false-color
image showing the observed infrared
sky brightness at wavelengths of 60
(blue), 100 (green) and 240 µm
(red). The bright white-yellow horizontal
band across the middle of the image
corresponds to emission from interstellar
dust in the plane of our Milky Way Galaxy
(the center of the Galaxy lies at the
center of the map). The red regions
above and below this bright band are
"infrared cirrus" clouds,
wispy clouds of relatively cool Galactic
dust. The blue S-shaped figure follows
the ecliptic plane and represents emission
from interplanetary dust in the solar
system. The map in the middle is a 60-100-240
µm false-color image depicting
the sky after the foreground glow of
the interplanetary dust has been modeled
and subtracted; this image is dominated
by emission from interstellar dust in
the Milky Way. After the infrared light
from our solar system and Galaxy
has been removed, what remains is a
uniform Cosmic Infrared Background.
This is illustrated in the bottom image,
which shows just the residual 240 µm
brightness. The line across the center
is an artifact from removal of the Galactic
light. The DIRBE team reports detection
of this cosmic background light also
at 140 µm, and has set limits
to its brightness at eight other infrared
wavelengths from 1.25 to 100 µm
(see Slide 22). Credit: STScI
OPO - PRC98-01; M. Hauser and NASA.

Slide
24: This image combines data from
the DIRBE obtained at infrared wavelengths
of 100, 140 and 240 µm - the longest
wavelengths measured by this instrument.
The sky brightness at these wavelengths
is represented respectively by blue,
green, and red colors in the image.
This image shows where there is more
material (appears brighter) and where
this material is coldest (appears redder).
The plane of the Milky Way Galaxy lies
horizontally across the middle of the
image with the Galactic center at the
center. Most of the infrared radiation
seen in this image originates from cold
dust (approximately 20 K, or 20 degrees
Centigrade above absolute zero) located
in clouds of gas and dust between the
stars in the Milky Way Galaxy. The wispy-looking
dust features are called "infrared
cirrus." The region of the Orion
Nebula with active star formation -
approximately 1,500 light years distance
from the Sun - appears on the right
of the image below the plane of the
Milky Way. Neighboring galaxies, the
Large and Small Magellanic Clouds, appear
as faint "blobs" below and
slightly to the right of the Galactic
center. Much of the picture appears
to be the same color, indicating that
there is not a large variation in the
dust temperature. Because the brightness
of the Solar System and Galaxy tends
to decrease with increasing wavelength,
these long wavelength DIRBE measurements
are particularly valuable for searching
for the cosmic infrared background.

Slide
25: Signal flow in the DMR instrument,
which was designed to detect and enable
the characterization of temperature
differences ("anisotropy")
in the
cosmic microwave background radiation.
The DMR design is similar to that used
in instruments flown on balloons and
aircraft. The receiver input is connected
alternately to two separate antennas
that point at different parts of the
sky. If the two parts of the sky are
not equally bright, the detected signal
will change slightly when the switch
is moved from one antenna to the other.
The entire apparatus is rotated to show
that the difference comes from the sky
and not from differences in the two
antennas. The DMR instrument has three
separate receiver boxes, one for each
wavelength (3.3, 5.7 and 9.6 millimeters).
Each box has two separate and independent
receivers tuned to the same frequency,
to improve the sensitivity of the measurement
and to protect against a failure. The
antennas in a pair are pointed 60 degrees
apart and 30 degrees away from the spin
axis of the COBE spacecraft. Each antenna
receives microwave light from a 7 degree
diameter beam. The
Microwave Anisotropy Probe (MAP),
a new NASA mission, will measure the
cosmic microwave background anisotropy
at ten times higher spatial resolution.

Slide
26: The 9.6 mm DMR receiver partially
assembled.

Slide
27: Early DMR sky maps depicting
data obtained from the independent ("A"
and "B") channels at each
of the three observed microwave wavelengths:
3.3, 5.7 and 9.6 mm (corresponding frequencies
are 90, 53 and 31.5 GHz, or thousand
MHz, respectively). Like the maps depicted
in slides 20 - 23, these maps were smoothed
with a 7 degree beam, yielding an effective
angular resolution of 10 degrees. Each
map is an all-sky Mollweide projection
in Galactic coordinates. The plane of
the Milky Way Galaxy is horizontal across
the middle of each map. The asymmetry,
or "dipole," that dominates
the appearance of these maps is a smooth
variation between a relatively warm
(bright) area in the upper right to
a relatively cool (faint) area in the
lower left. The dipole asymmetry is
due to the motion of the solar system
relative to distant matter in the Universe.
Although the signal attributed to this
variation is very weak - only one thousandth
the brightness of the sky - it is about
a hundred times stronger than the cosmic
microwave background anisotropy which
the DMR was designed to detect and so
must be subtracted before the anisotropy
can be seen.

Slide
28: Following subtraction of the
dipole anisotropy and components of
the detected emission arising from dust
(thermal emission), hot gas (free-free
emission), and charged particles interacting
with magnetic fields (synchrotron emission)
in the Milky Way Galaxy, the
cosmic microwave background (CMB)
anisotropy can be seen. CMB anisotropy
- tiny fluctuations in the sky brightness
at a level of a part in one hundred
thousand - was first detected by the
COBE DMR instrument. The CMB radiation
is a remnant of the Big Bang, and the
fluctuations are the imprint of density
contrast in the early Universe (see
slide 24 caption). This image represents
the anisotropy detected in data collected
during the first two years of DMR operation.
Ultimately the DMR was operated for
four years. See slide 19 caption for
information about map smoothing and
projection.

Slide
29: Maps based on 53 GHz (5.7 mm
wavelength) observations made with the
DMR over the entire 4-year mission (top)
on a scale from 0 - 4 K, showing the
near-uniformity of the CMB brightness,
(middle) on a scale intended
to enhance the contrast due to the dipole
described in the slide 19 caption, and
(bottom) following subtraction
of the dipole component. Emission from
the Milky Way Galaxy is evident in the
bottom image. See slide 19 caption for
information about map smoothing and
projection.

Slide
30: Maps based on observations made
with the DMR over the entire 4-year
mission, at each of the three measured
frequencies, following dipole subtraction.
See slide 19 caption for information
about map smoothing and projection.

Slide
31: The 53 GHz DMR sky map (top)
prior to dipole subtraction, (middle)
after dipole subtraction, and (bottom)
after subtraction of a model of the
Galactic emission, based on data gathered
over the entire 4-year mission. The
Galactic emission model is based on
DIRBE far-infrared and Haslam et
al. (1982) 408 MHz radio continuum
observations (see
Bennett et al. 1996, ApJ, 464,
L1). Bennett et al. excluded
an area around the Galactic plane referred
to as the "custom cut" region
when they conducted their analysis.
See slide 19 caption for information
about map smoothing and projection.

Slide
32: DMR "Map of the Early Universe."
This false-color image shows tiny variations
in the intensity of the cosmic microwave
background measured in four years of
observations by the Differential Microwave
Radiometers on NASA's Cosmic Background
Explorer (COBE). The cosmic microwave
background is widely believed to be
a remnant of the Big Bang; the blue
and red spots correspond to regions
of greater or lesser density in the
early Universe. These "fossilized"
relics record the distribution of matter
and energy in the early Universe before
the matter became organized into stars
and galaxies. While the initial discovery
of variations in the intensity of the
CMB (made by COBE in 1992) was based
on a mathematical examination of the
data, this picture of the sky from the
full four-year mission gives an accurate
visual impression of the data. The features
traced in this map stretch across the
visible Universe: the largest features
seen by optical telescopes, such as
the "Great Wall" of galaxies,
would fit neatly within the smallest
feature in this map. (See
Bennett et al. 1996, ApJ, 464,
L1 and references therein for details.)

Slide
33: Concept of FIRAS, showing light
from the sky being focused through cone
and sent to interferometer. The FIRAS
instrument was designed to measure precisely
the spectrum of the cosmic microwave
background radiation over a wavelength
range from 0.1 to 10 mm. The instrument
measures the wavelength of the incoming
radiation by using the phenomenon of
wave interference. A Michelson interferometer
is used to break the wave into two equal
parts, to delay one part, and then to
recombine them. The waves recombine
perfectly (constructive interference)
if the delay is a whole number of wavelengths,
but cancel perfectly (destructive interference)
if the delay is an odd number of half
wavelengths. The wavelength can be measured
by varying the delay and noticing where
the interference is constructive and
destructive. As the delay is changed,
the varying intensity at each detector
is called the interferogram. The interferogram
contains the information needed to determine
the intensity of the incoming light
at a large number of wavelengths (i.e.,
a spectrum) simultaneously. This method
provides an enormous advantage in sensitivity
over more direct methods. The FIRAS
uses a trumpet-shaped cone to collect
light from the sky and funnel it into
the instrument while rejecting light
that arrives from unwanted directions.
The field of view is 7 degrees, like
the DMR, so a spectrum can be obtained
at about 1000 independent parts of the
sky. The accuracy of the FIRAS is achieved
by a large blackbody calibrator which
can be inserted by command into the
mouth of the cone. (A blackbody is an
object that absorbs all radiation that
falls on it and radiates heat and light
with an intensity that can be computed
precisely if its temperature is known.
For a related tutorial, see
"About Temperature" by
Dr. Beverly Lynds.) The temperature
of the calibrator can be controlled
to within 0.001 K. The detectors are
thermometers which can easily detect
temperature changes caused by changes
of only a hundred trillionth of a watt
in the incident power.

Slide
36:
Cosmic microwave background (CMB)
spectrum. The solid curve shows the
expected intensity from a single temperature
blackbody spectrum, as predicted by
the hot
Big Bang theory. The FIRAS data
were taken at 34 positions equally spaced
along this curve. The FIRAS data match
the curve so exactly, with error uncertainties
less than the width of the blackbody
curve, that it is impossible to distinguish
the data from the theoretical curve.
These precise CMB measurements show
that at least 99.994% of the radiant
energy of the Universe was released
within the first year after the Big
Bang itself. All theories that attempt
to explain the origin of large scale
structure seen in the Universe today
must now conform to the constraints
imposed by these measurements. The results
show that the radiation matches the
predictions of the hot Big Bang theory
to an extraordinary degree. See
Mather et al. 1994, Astrophysical
Journal, 420, 439,"Measurement
of the Cosmic Microwave Background Spectrum
by the COBE FIRAS Instrument,"
Wright et al. 1994, Astrophysical
Journal, 420, 450,"Interpretation
of the COBE FIRAS CMBR Spectrum,"
Fixsen et al. 1996, Astrophysical
Journal, 473, 576,"The
Cosmic Microwave Background Spectrum
from the Full COBE FIRAS Data Sets,"
and
Mather et al. 1999, Astrophysical
Journal, 512, 511,"Calibrator
Design for the COBE Far Infrared Absolute
Spectrophotometer" for details.

Slide
37: FIRAS measured
cosmic microwave background radiation
residual spectrum from
Mather et al. 1994, ApJ, 420,
439. A Planck blackbody spectrum
and a small Galactic emission component
have been subtracted from the measured
spectrum in order to make the residuals
visible. To a very good approximation,
the cosmic microwave background spectrum
is the same as that of a 2.728 (+/-
0.004) degree Kelvin blackbody.

Slide
38: FIRAS map of C+ 158 µm and N+ 205 µm line intensity maps from
Fixsen
et al. 1999, Astrophysical Journal, 526, 207,"COBE Far Infrared Absolute Spectrophotometer Observations of Galactic Lines" . The maps are projections of the full sky in Galactic coordinates. The plane of the Milky Way
is horizontal in the middle of the map with the Galactic center at the center. The C+ line (top) is an important coolant
of the interstellar gas, in particular the "Cold Neutral Medium" (e.g., surfaces of star-forming molecular clouds).
In contrast, the N+ line emission (bottom) arises entirely
from the "Warm Ionized Medium" which surrounds hot stars. The maps are smoothed to 10° resolution. Color bars indicate emission intensity in units of nWm-² sr-¹. The scale is not linear.

Slide
40: A map of the temperature of
interstellar dust in the Milky Way Galaxy
derived from FIRAS sub-millimeter data.
The map is a projection of the full
sky in Galactic coordinates. The plane
of the Milky Way is horizontal in the
middle of the map with the Galactic
center at the center. At high frequencies,
the continuum in a FIRAS spectrum is
dominated by thermal dust emission;
at low frequencies, the cosmic microwave
background dominates. A single-temperature
dust model (with 1.55 adopted as the
emissivity spectral index) was used
to make this map. Different models can
be used and assumptions made, and corresponding
temperature and optical depth maps can
be derived straightforwardly from the
FIRAS Continuum Spectrum Maps (see
"About the Data Products"
in the FIRAS section of the
COBE Home Page). Reach et al.
(
1995, ApJ, 451, 188, "Far-Infrared
Spectral Observations of the Galaxy
by COBE"), for example, report
evidence for a ubiquitous cold (~5 K)
dust component.